Photomasks are used in photolithographic processes to form circuit patterns for integrated/printed circuits. As part of the photolithographic process, a photomask is used to image a two dimensional pattern corresponding to the circuit design on a substrate surface coated with a light-sensitive photoresist material. The imaging is accomplished by selectively exposing the photoresist to light directed through transparent areas in the photomask placed in contact with or in closely spaced relation to the substrate.
After the photoresist has been selectively exposed to light, a solvent may be used to remove portions thereof to expose the substrate therethrough. The exposed areas of the substrate surface may then be doped, etched or have materials deposited thereon. Once such processing has been completed the remaining photoresist is then removed by well known techniques.
Typically, a photomask comprises a patterned metal film such as chromium, nickel or aluminum having a thickness in the order of 500 to 1000 A. deposited on a transparent base such as quartz or glass having a thickness of 0.06 to 0.09 inch. The photomask is typically manufactured by depositing a thin film of the metal on the entire surface of the transparent substrate. The film of metal is then coated with a photoresist which is sequentially exposed, developed and the metal selectively removed in the exposed areas by etching, leaving a predetermined pattern of metal film on the substrate.
In the manufacture of photomasks, transparent defects such as pin holes or missing portions of metal film may occur. These defects, in turn, cause defective integrated circuits or other devices produced when using such masks. Since the manufacture of photomasks is generally a time consuming and relatively expensive operation, it is often desirable to repair a defective photomask rather than to discard it.
At present, a lift-off method frequently is used for repairing transparent defects on the photomask. This method is similar to the method of manufacturing the original photomask. The lift-off method includes the following steps: (1) applying a photoresist to the entire patterned surface of a photomask which includes the transparent defects; (2) exposing only the photoresist areas corresponding to the underlying transparent defects to light using a partial exposure method; (3) developing and removing the photoresist at the transparent defect areas in order to reveal the underlying defects; (4) forming a metallic film over the entire surface of the photomask including the transparent defects; (5) removing the photoresist and, at the same time, removing the metallic film formed thereon while (6) leaving the metallic film formed over the transparent defects.
There are several disadvantages with using this method. First, the photoresist which is applied in the first step may itself develop pin holes thus causing the metallic film, applied in step (4), to be deposited in areas other than the patterned regions. This creates what is known as opaque defects on the transparent substrate which then need to be corrected. Secondly, the metallic film which is deposited in the defect portions of the pattern may come off during lift-off, i.e., step (5). Finally, such a technique requires many steps and is time consuming.
Another method for repairing transparent defects is described in U.S. Pat. No. 4,340,654 which issued on July 20, 1982. This method utilizes laser-induced fusing of an opaque coating material to the photomask substrate surface at the location of the transparent defect. Undesirably, fusion causes physical and optical damage of the substrate and the deposited coating material. Should too large an area, or the wrong area, of the photomask substrate be exposed, it would be impossible to correct the newly formed opaque area with standard repair techniques since the underlying substrate is now damaged or removed.
Another method for repairing transparent defects on a photomask is described in U.S. Pat. No. 4,444,801 which issued on Apr. 24, 1984. This method utilizes laser-induced metal deposition and provides that a metal-organic complex solution be applied either to the entire surface of the photomask having transparent defects or only to the transparent defect portion and its periphery. Each transparent defect is then exposed to visible rays or ultraviolet rays so as to deposit the metal or a composition thereof into the transparent defect thereby correcting the defect on the photomask pattern. The exposure is halted or terminated when the deposition has been completed.
Experimentation demonstrates that mere exposure to a laser pulse of a metal-organic material that absorbs the radiation, converting it to thermal energy, and undergoes exothermic decomposition can result in an uncontrolled reaction producing an irregular temperature rise. This reaction is caused by conversion of the laser radiation to thermal energy. The energy released from the decomposition of the metal-organic material combined with the energy delivered by the laser results in irregular depositions. The subsequent temperature rise is so abrupt that the organic component of the material becomes charred and an undesired non-volatile, non-opaque, non-adherent residue is formed with the deposited metal.
Accordingly, there is a need for an effective transparent mask defect repair technique which provides adequate adhesion of the deposited metal without concomitant charring of the metal precursor.